In the poker variant of Texas Hold 'em, Rockets refers to having two aces as your hole cards. This is usually seen as the holy grail of hold 'em hands and wins a very large majority of the time. Yet, seen by many neophytes as a terrible hand because 'they always get drawn out'. Chances you receive aces as your hole cards are .45%.

The first usage is derived from the second. The term "Rock-It" has long been used to describe the act of man having sex with a woman. Where the man in "rockin" the woman. The second term is used to say that a woman has a body that is worth "rocking".

1) Dan, look at Lizzy, she has a rocket body.
Meaning: Dan, look at Lizzy, she is a very attractive woman.

2) Lizzy is hot. I am telling you Dan, I just want to rocket.
Meaning: Dan, I want to have sex with Lizzy because she is hot.